1. Field of the Invention
The present invention pertains to curing of unsaturated polyester resins in admixture with ethylenically unsaturated copolymerizable monomers and is particularly concerned with promoting or accelerating the cross linking of such polyester with such vinyl monomers during curing while retaining serviceable shelf-life during storage of the permix at ambient or room temperatures.
2. The Prior Art
The copolymerizable of unsaturated polyester resins with vinyl monomers in the presence of free-radical generating compounds such as organic peroxides is well known in the art. Among the vinylic monomers suggested or employed are: methyl methacrylate, vinyl chloride, vinyl acetate, vinyl toluene, and styrene; the last named being largely preferred. The unsaturated polyesters are obtained by reacting one or more dihydric alcohols with an unsaturated dicarboxylic acid as cis-butenedioic acid (maleic acid) or its anhydride or trans-butenedioic acid (fumaric) or mixtures of these, sometimes in the presence of a saturated dicarboxylic acid such as phthalic or isophthalic. The monomer may comprise 30 to 70% by weight of the admixture with the unsaturated polyester.
A wide range of organic peroxide catalysts, called "initiators," are known and have been commercially employed for the polymerization of unsaturated polyesters and their copolymerization with vinyl monomers. These function by way of undergoing cleavage of the peroxide linkage to form highly active free radicals which initiate the polymerization reaction. These peroxide initiators vary widely as to the temperature at which they initially undergo cleavage and as to the rate of decomposition to provide free radicals at any given temperature. Accelerators, also called promoters, are frequently used to speed up free radical formation from the peroxide at lower temperatures, particularly in connection with formulations intended to be cured at or about room temperature.
A wide use of unsaturated polyester-styrene copolymer systems is in sheet molding compound (SMC) and bulk molding compound (BMC) which comprise the resin reinforced with glass fibers and which generally contain fillers, a lubricant and thickeners. These systems employ initiators that decompose at higher temperatures and further necessitate longer term storage stability than the general purpose resins. While by the inclusion of effective promoters the gel and cure time of such fibrous glass reinforced plastics can be accelerated, these accelerators generally have an adverse effect on required storage stability of the uncured system. Moreover, most BMC and SMC formulations also include a thermoplastic resin, such as an acrylic polymer, to reduce shrinkage during cure. These acrylic polymers are incompatible with the unsaturated polyester and phase rigidly after blending, often resulting in an undesirable mottled surface of the cured product particularly accentuated when the cure is accelerated.
Benzoyl peroxide was at one time the most widely used initiator for polyester molding compositions such as those empolyed in BMC and SMC formulations. However, because this "hot initiator" diminishes the shelf life of these molding compositions even in the presence of commonly employed inhibitors, it has been supplanted in many molding plants by t-butylperbenzoate. More recently, peroxyketal initiators have been introduced as initiators for polyester resin compositions such as BMC and SMC. Among the better known of these peroxyketal initiators in commercial use are 1,1 bis(t-butyl peroxy) cyclohexane and 1,1-di-t-butylperoxy-3,3,5-trimethyl cyclohexane.
To inhibit premature gel formation of polyester resin compositions, various types of inhibitors are generally employed to improve shelf life; the more widely used inhibitors being quinone, hydroquinone or other phenolic types. Quaternary ammonium salts have also been described as efficient inhibitors of gelation during storage of polyester resin compositions. See: Unsaturated Polyesters: Structure and Properties, H. V. Boening, Elsevier, New York (1964), pages 45, 46; and Polyesters and Their Applications by Bjorksten Research Labs, Inc., Rheinhold, New York (1956), pages 53, 54, 172. Most commonly, uncured commercial polyester resin compositions contain 0.01 to about 0.1% or more of an inhibitor of the quinonic or phenolic type to inhibit premature internal polymerization.
In U.S. Pat. Nos. 2,593,787 and 2,627,510 polymerizable polyester resin compositions are described which are stabilized against premature gelation during prolonged periods of storage by incorporation of certain quaternary ammonium salts. The latter patent employs the quaternary ammonium salt in combination with chloranil in such polyester compositions containing a peroxide initiator.
According to U.S. Pat. No. 3,028,360, the stabilizing effect of quaternary ammonium salts and other named inhibitors, is stated to be improved by further incorporation into the unsaturated polyester resin composition of a small amount of a copper salt.
In U.S. Pat. No. 3,288,735, it is stated that unsaturated polyester resins containing free radical polymerization catalyst, may be maintained for extended periods in gel state by admixture into the resin composition of certain types of additives prior to gel formation. Among the various named additives for this purpose is a mixture of a quaternary ammonium compound and a mercaptan.
While recognizing that certain salts of quaternary ammonium compounds are valuable gelation inhibitors in mixtures of unsaturated polyester resins and interpolymerizable monomers, U.S. Pat. No. 2,740,765 indicates that when such interpolymerizable mixtures contain certain peroxide or hydroperoxide initiators and are stabilized with a quinonic or phenolic stabilizer, further incorporation therein of a quaternary ammonium salt accelerates gelation.
In systems employing a redox initiator composed of peroxides and hydroperoxides with sulfur-containing reducing compounds, U.S. Pat. No. 2,946,770 suggests the use of a quaternary ammonium compound in combination with a copper salt, to accelerate polymerization time.
While accelerators or promoters are more usually employed in connection with peroxide initiators in resin formulations intended to be cured at or about room temperature, the use of promoters is not limited thereto. Not all peroxide initiators, however, can be activated by known promoters (Boening, ibid page 46). Among the free radical initiators that can be promoted are ketone peroxides, such as MEK peroxide, benzoyl peroxide, t-butyl peroctoate and t-butyl perbenzoate. Among the best known promoters are metal compounds (primarily those of cobalt such as cobalt naphthenate) and certain amines, particularly dimethyl aniline. For t-butylperbenzoate the preferred promoter is cobalt naphthenate. While dimethyl aniline is known to accelerate benzoyl peroxide at room to moderate temperatures, cobalt naphthenate does not do so.
While under certain conditions and particularly in specific combinations with heavy metal compounds or other accelerating adjuvants, quaternary ammonium compounds have been reported to accelerate copolymerization of unsaturated polyester resins with vinyl type monomers in the presence of certain peroxide or hydroperoxide free radical initiators, these quaternary ammonium compounds have been found ineffective as promoters in BMC and SMC cured at high temperature with benzoyl peroxide or t-butyl perbenzoate. While the curing of such unsaturated polyester molding compositions employing t-butyl perbenzoate initiator can be accelerated by heavy metal compounds such as organic cobalt compounds, the extent of such acceleration is diminished in the presence of quaternary ammonium salts. On the other hand, such cobalt compounds are ineffective in unsaturated polyester molding compositions cured at elevated temperature with peroxyketal initiators. In fact, prior to the present invention, no effective means for accelerating the decomposition of peroxyketal initiators was known to the art.